Ion-exchanged glass sheets, also known as chemically-strengthened or ion-strengthened glass sheets, are used in a variety of applications. For example, ion-exchanged glass sheets are widely used as touch screens for hand-held consumer electronics such as smart phones and tablets. Perhaps the best-known example of an ion-exchanged glass sheet is the scratch-resistant faceplate of the iPhone® made from Corning Incorporated's Gorilla® Glass.
In broad overview, ion-exchanged glass sheets are made by forming a glass having a composition suitable for chemical strengthening into a glass ribbon from which individual glass sheets are cut, and then subjecting the glass sheets to chemical strengthening through an ion-exchange (IOX) process, e.g., a treatment in which the glass sheet is submersed in a salt bath at an elevated temperature for a predetermined period of time.
The IOX process causes ions from the salt bath, e.g., potassium ions, to diffuse into the glass while ions from the glass, e.g., sodium ions, diffuse out of the glass. Because of their different ionic radii, this exchange of ions between the glass and the salt bath results in the formation of a compressive layer at the surface of the glass which enhances the glass's mechanical properties, e.g., its surface hardness. The effects of the ion exchange process are typically characterized in terms of two parameters: (1) the depth of layer (DOL) produced by the process and (2) the final maximum surface compressive stress (CS). Values for these parameters are most conveniently determined using optical measurements, and commercial equipment is available for this purpose, e.g., instruments sold by Frontier Semiconductor and Orihara Industrial Company, Ltd.
Although glass sheets can be produced by a variety of glass-sheet manufacturing processes, the two main processes that are currently in commercial use to produce glass sheets that are to be subjected to an IOX process are the float process and the overflow downdraw fusion process (hereinafter referred to as the “fusion process”). The present disclosure will thus focus on these processes, it being understood that the methods for quantifying Z-axis asymmetries disclosed herein are also applicable to other glass-sheet manufacturing processes now known or subsequently developed.
In the case of the fusion process, a glass ribbon is formed by passing molten glass around the outside of a forming structure (known in the art as an “isopipe”) to produce two layers of glass that fuse together at the bottom of the forming structure (the root of the isopipe) to form the glass ribbon. The glass ribbon is pulled away from the isopipe by pulling rollers and cooled as it moves vertically downward through a temperature-controlled housing. At, for example, the bottom of the housing (bottom of the draw), individual glass sheets are cut from the ribbon. In the case of the float process, a glass ribbon is formed on the surface of a molten metal bath, e.g., a molten tin bath, and after being removed from the bath is passed through an annealing lehr before being cut into individual sheets.
In both processes, the glass ribbon and/or the glass sheets cut from the ribbon are exposed to conditions that may be asymmetric with respect to the front and back sides of the glass and thus may affect the results of an ion-exchange process subsequently applied to the glass sheets. For example, the process of removing glass sheets from a glass ribbon is normally asymmetric, with scoring taking place on only one side of the ribbon followed by separation of the glass sheet by rotation in a direction which opens the score line (i.e., if the score line is formed in the front surface of the glass ribbon, then the rotation takes place towards the back surface as seen from the side of the ribbon). See, for example, Andrewlavage, Jr., U.S. Pat. No. 6,616,025.
Because sheet removal in the float process normally occurs after the ribbon has passed through an annealing lehr and because sheets are cut from the ribbon while the ribbon is supported horizontally, the conditions associated with sheet removal normally do not constitute ion-exchange-affecting conditions for the float process. However, for the fusion process, the glass ribbon hangs vertically and forces applied to the ribbon during the sheet removal process can propagate upward through the ribbon thus affecting the ribbon's position and shape at locations where, for example, the glass is passing through its glass transition temperature range. Consequently, conditions associated with sheet removal can constitute asymmetric ion-exchange-affecting conditions for the fusion process.
Various approaches have been disclosed for reducing movement of a glass ribbon and/or changes in the ribbon's shape during sheet removal in the fusion process. See, for example, Chalk et al., U.S. Pat. No. 7,895,861; Abbott, III et al., U.S. Patent Application Publication No. 2006/0042314; Kirby et al., U.S. Patent Application Publication No. 2007/0095108; and Kirby et al., U.S. Patent Application Publication No. 2010/0043495. Moreover, in some situations, especially when dealing with thin and/or wide glass ribbons, it may be desirable to intentionally introduce a bow into the glass ribbon. See, for example, Burdette et al., U.S. Patent Application Publication No. 2008/0131651; and Burdette, U.S. Pat. No. 8,113,015. Such bowing can cause the concave and convex sides of the ribbon to experience somewhat different thermal conditions as the ribbon moves through the fusion draw machine. Such different thermal histories for the two major surfaces of a glass sheet can constitute asymmetric ion-exchange-affecting conditions for a fusion process.
As noted above, the float process does not normally suffer from asymmetries associated with sheet removal. However, the float process has a basic asymmetry that arises from the fact that only one surface of the glass ribbon contacts the molten metal bath. This asymmetry is known to result in different ion-exchange properties for the two sides of the sheet. In particular, after undergoing an IOX process, glass sheets produced by the float process exhibit warp, with the surface that was not in contact with the molten metal bath becoming a convex surface. See, for example, U.S. Pat. No. 4,859,636.
Various process steps have been added to the basic float process so that the ion-exchange-affecting conditions for the overall glass-sheet manufacturing process are less asymmetric. For example, the above-referenced U.S. Pat. No. 4,859,636 adds the step of contacting the surface of the glass sheet that was in contact with the molten metal with a source of sodium ions prior to ion-exchange strengthening, while PCT Patent Publication No. WO 2012/005307 and U.S. Patent Application Publication No. US 2012/0196110 respectively describe plasma treating and forming a film of SiO2, TiO2, NESA, ITO, AR, or the like on one of the surfaces of the glass sheet prior to ion exchange strengthening. In addition to adding process steps, changes to the basic float process with or without additional process steps, have also been disclosed. See International Publication Numbers WO 2013/005588 and WO 2013/005608.
A common characteristic of the approaches used to deal with asymmetric ion-exchange-affecting conditions of a glass-sheet manufacturing process, whether it be a float process or a fusion process, has been their ad hoc nature. Put simply, other than through trial and error or complex and difficult measurements performed on glass samples (see, for example, International Publication Number WO 2013/005588), there has been no way to quantify whether a particular manufacturing process has or does not have sufficient Z-axis symmetry to be suitable for producing glass sheets that are to be subjected to an IOX process.
This is especially so when it is considered that IOX processes in and of themselves have numerous variables (e.g., time-temperature profiles, bath compositions, use of multiple ion-exchange stages each with its own time-temperature profile and bath composition, etc.). The ad hoc process is thus even more challenging when trying to determine if a particular manufacturing process or particular additions to a basic manufacturing process (e.g., the addition of a plasma treatment) will provide a sufficiently low level of Z-axis asymmetry for more than one possible IOX treatment of the glass sheets produced by the process. Along these same lines, it has not been possible to predict whether a particular manufacturing process or particular additions to a basic manufacturing process provide sufficient Z-axis symmetry to be used in producing a specified product, e.g., glass sheets having a specified size/thinness combination.
As discussed fully below, in accordance with certain of its aspects, the present disclosure addresses the above-described deficiencies in the art by providing methods for measuring (quantifying) the intrinsic (native) Z-axis asymmetry of a glass-sheet manufacturing process. The quantification is in terms of an asymmetry value, i.e., an “ASYM” value, which is a dimensionless number, or a BM1 value, which has the dimensions of distance. Using either or both of these asymmetry values, comparisons can be made between a given glass-sheet manufacturing process operated under various conditions or between glass-sheet manufacturing processes of various types.